1. Field of the Invention
The present invention relates to an image forming apparatus of a laser scanning type.
The apparatus of the present invention improves the timing precision of laser beam detection system by stabilizing the amount of laser light to be detected by the laser beam detecting system (start-of-scan (SOS) sensor) prior to detection.
2. Description of the Related Art
Laser printers, laser COM systems and the like are provided with devices for forming images by laser scanning.
The aforesaid devices use various control methods which precisely align the image front edge position of each scan line, detect the laser beam at a predetermined position before the start of scanning and start the laser beam output based on the line image data after a predetermined period has elapsed from the time of the laser beam detection so as to produce superior images without image jitter.
The amount of light of a laser beam emitted by a semiconductor laser is affected by heat characteristics called droop characteristics.
The aforesaid droop characteristics are shown in interval T1 of FIG. 1a. That is, the amount of light is greater (amount of light: Lh0) immediately after the semiconductor laser is turned on due to the comparatively low temperature, and is gradually reduced as the temperature rises until the light is eventually stabilized at a constant value (L1). The light quantity Lh0 may be several fold greater than L1.
As shall be fully described later, the aforesaid interval T1 is a time zone for forcing the semiconductor laser to switch on to allow detection of the laser beam by the aforementioned SOS sensor. The interval T1 is hereinafter referred to as "forced beam generating interval T1."
In FIG. 1a, no image data are present between the first forced beam generating interval T1 (left side) and the second forced beam generating interval T1 (right side). That is, the semiconductor laser is set in the off state. Thus, the semiconductor laser temperature decreases between the first forced beam generating interval T1 and the second forced beam generating interval T1. Accordingly, the initial amount of light of the second forced beam generating interval T1 is identical to the initial amount of light of the first forced beam generating interval T1, i.e., the high value Lh0.
At this time, the amount of light of the detected laser beam exhibits no significant fluctuation from that of the first laser beam detected in the forced beam generating interval T1, as shown in FIG. 1b.
On the other hand, when image data are present in the interval T4 between the first forced beam generating interval T1 (left side) and the second forced beam generating interval T1 (right side), the semiconductor laser is turned on and off in accordance with said image data so that the temperature of the semiconductor laser does not decrease during the image signal interval T4. Therefore, the initial amount of light of the second forced beam generating interval T1 is a value Lh1 which is lower than the initial amount of light of the first forced beam generating interval T1.
At this time, the amount of light of the detected laser beam fluctuates markedly from that of the first laser beam detected in the forced beam generating interval T1 due to the effects of droop characteristics, as shown in FIG. 1d.
In a general case, image data are present in image signal interval T4 like FIG. 1c.
The laser beam detection time as detected via the SOS sensor (the standard period for starting the clock for image data output) is determined as the period wherein the SOS sensor output exceeds a predetermined threshold level .phi., so that laser beam output based on the line image data starts after a predetermined time period has elapsed after the aforesaid beam detection period. That is, control is executed to precisely align the image front edge position of each scan line so as to prevent image jitter.
Accordingly, when the amount of laser light is high (broken line: Lh0) and low (solid line: Lh1), as indicated by ta and tb in FIG. 2, the aforementioned standard period produces a slight dislocation which in turn causes dislocation of the laser beam output starting position based on the line image data of each scan line, thereby resulting in image jitter.
In the previously described forced beam generating interval T1, the initial laser beam of the interval T1 has a light emission which is easily affected by fluctuations in the temperature of the semiconductor laser. The laser beam is detected by the SOS sensor, as shown in FIG. 1d, and the standard period is determined as shown in FIG. 2. When control is executed in this manner, discrepancies are produced in the standard period causing image jitter.
U.S. Pat. No. 4,264,120 discloses a method for eliminating the previously described disadvantages wherein a stabilized light quantity can be detected via an SOS sensor at the end of a predetermined forced beam generating interval T1 so as to improve the precision of the reference period thereby.
In high-speed laser printers, however, it is currently impossible to set the forced beam generating interval T1 so as to be long enough for the beam light quantity to stabilize by the end of said interval T1. That is, it is currently impossible to adapt the method disclosed in U.S. Pat. No. 4,264,120 to high-speed laser printers.